Plasmon polaritons have revolutionized our world of nanophotonics. They have created a platform for enhanced light-matter interactions, propagation of light beyond the diffraction limit, and nanofocusing of electromagnetic energy. However, for applications in data processing and telecommunication, dissipation of optical energy in metallic waveguides is much beyond what can be tolerated for nanocircuitry. Recently other groups of polaritonic waves, namely photon polaritons, exciton polaritons, and Dirac plasmons have been demonstrated as possible candidates for nanophotonics1. Interestingly, a topological insulator like Bi2Se32, as well as heterostructures like graphene/hBN 3, can support coexisting polaritonic waves of all the kinds stated above. At THz frequencies and near to the Fermi energy level, those materials support both hyperbolic phonon polaritons and Dirac plasmons, whereas at infrared and visible ranges4,5 exists another channel for exciton-polariton mode.
Here, we mainly discuss the dispersion of the surface polaritons and their spatiotemporal behaviors, at all the energy ranges stated above. We however mainly focus on an aspect of topological insulators which is less discussed beforehand, i.e. topological magnetoelectric effect6. We study the criteria for existence of propagating optical modes which are transversely bound at the interface of two materials. In particular, quite general cases are considered, where the materials involved are assumed to be anisotropic, but also demonstrating magneto-electric effects7. We also discuss the situations where the coexistence of Dirac and hyperbolic polaritons result in level repulsion. We further study the effect of topological magnetoelectric effect on the appearance of hybrid optical modes with various polarization states.
In addition to surface polaritons, existence of wedges support another channel for long range propagation of hyperbolic polaritons, due to the coupling of two edge polaritons. We study here the behavior of hyperbolic wedge polaritons at visible and ultraviolet energy ranges. We discuss the radiation damping and long range propagation of hyperbolic wedge and surface polaritons, both theoretically and experimentally using electron energy-loss spectroscopy and finite-difference time-domain method.
References
1 Basov, D. N., Fogler, M. M. & de Abajo, F. J. G. Polaritons in van der Waals materials. Science 354, aag1992 (2016).
2 Wu, J.-S., Basov, D. N. & Fogler, M. M. Topological insulators are tunable waveguides for hyperbolic polaritons. Phys. Rev. B 92, 205430 (2015).
3 Woessner, A. et al. Highly confined low-loss plasmons in graphene-boron nitride heterostructures. Nat. Mater. 14, 421-425 (2015).
4 Esslinger, M. et al. Tetradymites as Natural Hyperbolic Materials for the Near-Infrared to Visible. Acs Photon. 1, 1285-1289 (2014).
5 Talebi, N. et al. Wedge Dyakonov Waves and Dyakonov Plasmons in Topological Insulator Bi2Se3 Probed by Electron Beams. Acs Nano 10, 6988-6994 (2016).
6 Dziom, V. et al. Observation of the universal magnetoelectric effect in a 3D topological insulator. Nat. Commun. 8 15197 (2017).
7 Talebi, N. Optical modes in slab waveguides with magnetoelectric effect. J. Opt.-Uk 18, 055607 (2016)

Metal nanoparticles demonstrate unique optical properties that are mostly due to localized surface plasmon resonances (LSPRs). In addition, when nanoparticles are arranged in arrays (metasurfaces), their responses can be modified by the presence of the neighboring particles. As a result, sharp spectral features can be observed. Such features, called surface lattice resonances (SLRs), are related to the appearance of diffraction orders in the optical response. Both types of resonances can lead to local-field enhancement and thereby boost nonlinear optical effects. For the particular case of second-harmonic generation (SHG) the sample needs to be also non-centrosymmetric. This condition is fulfilled when, for example, V-shaped nanoparticles are used in the array. Increasing the number of particles typically increases the optical density, which should increase the nonlinear response with the square of the particle density. This approach, however, has its limitations because, when the particles are too close to each other, the quality of the LSPRs decreases leading to an effect opposite to the desired. Here, we will show the counterintuitive effect that the nonlinear response can be enhanced by reducing the number of particles in the array.
In order to verify our idea, we use two arrays of V-shaped gold nanoparticles fabricated on a glass substrate by electron-beam lithography and lift-off methods. The particles are distributed in 500 x 500 nm2 square arrays in two configurations: i) all lattice points are filled with particles (V1) or ii) every other particle in the lattice is removed in a way that the remaining particles form a rotated (by 45°) square array with a pitch of 707 nm (V2). Both samples have two eigenpolarizations: one along the symmetry axis (y) of the V shape and other in the perpendicular direction (x).
In the SHG experiments, the incident beam from an optical parametric oscillator was incident on the sample. Polarizers and a half-wave plate were used to control the polarization of the fundamental (1000 – 1300 nm) and second-harmonic beams. The SHG signal was collected by a photon counting system.
The sample V2, that has reduced (by a factor of 2) density of particles in the array, shows the expected decrease in the strength of the resonance peak (1151 nm) and a slight redshift of the resonance wavelength with respect to the sample V1 (1081 nm). In order to achieve fair comparison of the nonlinear signals, we tuned the incident wavelength to the position of approximately equal losses for both samples (1135 nm). The sample V2 is found to have, by a factor of 7, stronger response than sample V1. Such enhancement in the nonlinearity is related to the improvement in the quality of the resonance for sample V2, for which the width of the resonance is reduced by ~30% compared to V1. This is due to SLRs that are present for sample V2. Our results are in good agreement with calculations by using an approach based on the discrete-dipole approximation.

A gap plasmon is an electromagnetic wave propagating in a gap between two noble metal surfaces. Such gap plasmons have previously been studied using only a classical description of the noble metals, but this model fails and shows unphysical behavior for sub-nanometer gaps. To overcome this problem quantum spill-out is included in this paper by applying Density-Functional Theory (DFT), such that the electron density is smooth across the interfaces between metal and air. The mode index of a gap plasmon propagating in the gap between the two metal surfaces is calculated from the smooth electron density, and in the limit of vanishing gap width the mode index is found to converge properly to the refractive index of bulk metal. When neglecting quantum spill out in this limit the mode index shows unphysical behavior and diverges instead.
The mode index is applied to calculate the reflectance of an ultrasharp groove array in silver, as gaps of a few nm are found in the bottom of such grooves. At these positions the gap plasmon field is highly delocalized implying that it mostly exists in the bulk silver region where absorption takes place. Surprisingly, when the bottom width
is a few nm and the effect of spill out at a first glance seems to be negligible, strong absorption is found to take place 1-2 Å from the groove walls as a consequence of the dielectric function being almost zero at these positions. Hence quantum spill out is found to significantly lower the reflectance of such groove arrays in silver.

Epsilon-near-zero materials are ideal platforms for nonlinear optics. Extreme electric field enhancements are predicted when a transverse-magnetic polarized field impinges obliquely on a film of material whose real part of the dielectric permittivity approaches zero. Under these circumstances, the component of the electric field with polarization normal to the film surface is enhanced by a factor proportional to the inverse square root of the dielectric permittivity. Nonlinear processes benefit from such uniquely favorable field localization, whether the condition is achieved in natural or artificial materials. Nonlinear optical processes have also been shown to be affected by the interference mechanism that occurs when two counter-propagating beams/pulses interact. Counter-propagating pulse dynamics has been investigated for surface plasmons and guided beams, they have been used for direct characterization of ultra-short pulses, to control emission of high-harmonics and to indirectly measure the phase mismatch of waveguides. Finally, they have been studied also in one dimensional photonic crystals and negative index materials. However, all these examples rely on either phase-matching or the availability of photonic resonances. Here we demonstrate that, thanks to the ability of epsilon-near-zero materials to efficiently support nonlinear processes in the absence of phase-matching or resonant conditions, one can control harmonic generation process by altering the phase of two non-collinear counter-propagating beams. We investigate the dynamics of two non-collinear counter-propagating beams impinging on an epsilon-near-zero slab and evaluate the modulation of the second and third harmonic signals as a function of the phase difference between the two sources. The calculations are performed considering a 100nm-thick slab of indium-tin-oxide (ITO). The results confirm that epsilon-near-zero media are exceptional platforms for nonlinear optics, providing a novel path to control these processes, including the possibility to easily characterize optical pulses.

Nanoparticles exhibit various optical properties arising from scattering and absorption due to polariton excitation. The resulting frequency and amplitude is dependent on several factors such as particle size, shape, and dielectric environment. By modifying the environment of the nanoparticle surface, in particular by encapsulating an individual nanoparticle within a membrane bilayer comprising defined phospholipids, these properties may be utilised to interrogate molecular interactions adjacent to the particle surface to useful levels of sensitivity. We describe the underlying rationale of these properties and characterise the preparation and behaviour of the nanoparticles. We indicate the potential this approach may have for sensing and screening in analytical biomolecular technology by demonstrating that it can be utilised to reveal the kinetics of the molecular interactions of membrane associated events. We also indicate that the technique may yield higherorder structural information of the macromolecule-membrane interactions in a highly sensitive manner and discuss the physical origins of these potentially more exotic phenomena.

The development of planar functional junction provides continuous, single-atom thick, in-plane integrated circuits. The production of atomic contacts of different materials (hetero/homostructures) is still a challenging task for 2D materials technology. In this paper we describe a new method of formation of a photosensitive junction by femtosecond laser pulses patterning of graphene FET. The laser-induced oxidation of graphene goes under high intensity laser pulses, which provide nonlinear effects in graphene like multiphoton absorption and hot carrier generation. The process of laser induced local oxidation is studied on single-layer graphene FET produced by wet transfer of CVD grown graphene on copper foil onto a Si/SiO2 substrate. The 280 fs laser with 515 nm wavelength with various pulse energies is applied to modify of local electrical and optical properties of graphene. Thus, the developed process provides mask-less laser induced in-plane junction patterning in graphene. The scale of local heterojunction fabrication is about 1 μm. We observe that with an increasing of the laser fluence the number of defects increases according to two different mechanism for low and high fluences, respectively. The change of the charge carrier concentration causes the Dirac point shift in produced structures. We investigate the photoresponse in graphene junctions under fs pulsed laser irradiation with subthreshold energies. The response time is rather high while relaxation time is large because of charge traps at the graphene/SiO2 interface.

We propose octagonal quasi-crystal designs providing effective light confinement for different resonance frequencies through the structural modification with the utilization of low-symmetric photonic unit cells. The effect of rotational symmetry reduction on the cavity resonance appearing in the corresponding photonic bandgap of each structure has been investigated. Relatively small dielectric cylinders have been additionally located at discrete angular positions with particular distances from the center of the each core cylinder and the noteworthy resonance peaks have been observed to emerge in the bandgaps. Rotational symmetry of the proposed structures is to be modified by varying the angular displacement of the smaller quasi-crystalline rods with the angle θ in terms of the x-axis of the small rod. The successful demonstration of tunable resonance modes has been achieved numerically and experimentally for the first time by tailoring the positional parameters and reducing the crystalline symmetry. Strongly localized modes in the proposed quasi-crystals have great potential for various slow light applications along with other technologies such as sensors, lasers and memory units.

We pursue a nanophotonic platform for strong light-matter interaction that combines plasmonic mode volumes, i.e., deep subwavelength confinement, with cavity quality factors (Q = 1000 to 100000). To this end we study the physics of resonator structures in which plasmon antennas are placed inside microcavities, like microdisks and photonic crystal cavities. Coupled oscillator theory for the local density of optical states in such systems shows a rich family of Fano-type line shapes, meaning that interferences lead to both transparency windows (very low LDOS, even when both antenna and cavity are separately on resonance) and to Purcell factors that far exceed those of antenna and cavity alone. These results are further confirmed by full-wave modelling.
We will report experiments that probe the system from several viewpoints. First, we show that it is not true, even for high-Q cavities, that plasmon scatterers necessarily reduce Q, as evident from probing the cavity response in an experiment where we approach a cluster of plasmon nanorods to a microtoroid with a Q of 10^6. Second, we show that the polarizability of an antenna is strongly dependent on whether it is coupled to a microcavity, as evident from antenna extinction in the same experimental system. Thirdly, we show that in Si3N4 microdisk-antenna structures made by lithography that we decorate with single nanoantennas as well as phased arrays, dominantly plasmonic modes can be obtained even at Q’s well above 10.000. The richness of the physics that is evident from the experiments clearly goes well beyond simple perturbation models. The underlying mechanism is that both the cavity and the antenna are essentially open systems that have radiation as their main loss mechanism. Interaction and interference through these radiative channels leads to unexpected performance characteristics for light-matter interaction that in terms of coupled mode theory map on non-hermitian coupled oscillator properties. We believe that this can be captured by casting the problem in language of Quasi-Normal Modes. Our current efforts are devoted to matching these systems to near-infrared quantum emitters such as dibenzo-terrylene in anthracene for low-temperature quantum optics studies.

Hybrid nanomaterials are targeted by a rapidly growing group of nanooptics researchers, due to the promise of optical behavior that is difficult or even impossible to create with nanostructures of homogeneous composition. Examples of important areas of interest include coherent coupling, Fano resonances, optical gain, solar energy conversion, photocatalysis, and nonlinear optical interactions. In addition to the coupling interactions, the strong dependence of optical resonances and damping on the size, shape, and composition of the building blocks provides promise that the coupling interactions of hybrid nanomaterials can be controlled and manipulated for a desired outcome. Great challenges remain in reliably synthesizing and characterizing hybrid nanomaterials for nanooptics.
We review and describe the synthesis, characterization, and applications of new hybrid plasmonic nanomaterials that are created through plasmon-induced photopolymerization. Involved polymer can contain active species, resulting in advanced hybrid nano-emitters
The work is placed within the broader context of hybrid nanomaterials involving plasmonic metal nanoparticles and molecular materials placed within the length scale of the evanescent field from the metal surface. We specifically review three important applications of free radical photopolymerization to create hybrid nanoparticles: local field probing, photoinduced synthesis of advanced hybrid nanoparticles (including light-emitting nanosystems), and nanophotochemistry.
We first demonstrate that nanoscale photopolymerization is possible at the surface of Ag nanoparticles,1,2 gold nanocubes3 and within the gap between two coupled metal nanoparticles.4 This local polymer integration enables symmetry breaking, quantification of plasmonic near-fields and trapping of molecules whose Raman signature gets amplified.
Secondly, we show that it is possible to integrate quantum nanoemitters in the vicinity of plasmonic nanostructures with high spatial precision via two-photon polymerization. 5 In particular, we demonstrate two-color nanoemitters that enable the selection of the dominant emitting wavelength by varying the polarization of excitation light. The nanoemitters were fabricated by using two polymerizable solutions with different quantum dots, emitters of different colors can be positioned selectively in different orientations in the close vicinity of the metal nanoparticles. The dominant emission wavelength of the metal/polymer anisotropic hybrid nanoemitter thus can be selected by altering the incident polarization.
1. Phys. Rev. Lett. 98, 107402 (2007)
2. ACS Nano 4, 4579 (2010)
3. J. Phys. Chem C. 116, 24734 (2012)
4. ACS Photonics 2, 121 (2015)
5. Nano Letters 15, 7458 (2015)

When two sub-wavelength metallic nanoparticles, each of them supporting a resonant plasmon, are brought within few nanometers or less from each other, the two plasmonic resonances are strongly coupled. The new eigenmodes of the system include in particular a dipolar mode, for which the maximum electric field is localized in the nanoscale gap between the particles. The local field enhancement compared to the incoming far field can be several hundred folds.
We present the design and fabrication of such plasmonic gap cavities, created by depositing gold nanospheres on an atomically flat gold surface, which has been functionalized with a self-assembled monolayer of thiol molecules. This system enables extremely large and reproducible enhancement of the Raman signal from the molecules.
Although these nanogap cavities have been used in SERS studies for some time already, a detailed understanding of the out-of-equilibrium physics under laser irradiation is missing. What is the local temperature of the electrons in the metal? Is the molecular vibration in equilibrium with the surrounding thermal bath?
We will present our latest results in the spectroscopy of these nanocavities under broadly tunable excitation. In particular, we want to clarify if a suitable detuning of the laser from the plasmonic resonance can lead to amplification of molecular vibrations [1] well above the thermal occupancy.
[1] P. Roelli et al, Nature Nanotechnology 11, 164–169 (2016)

A dynamic all-optically controlled surface plasmon polartions (SPP) novel high-performance multi-function optical microscope, combining optical microscopic imaging, bio-sensing and surface enhanced Raman Scattering (SERS) in a single microscopic system, is presented in this talk. This new configuration uses phase shift of SPP standing wave generated from sub-wavelength slit arrays embedded in a thin metal film to achieve super-resolution wide-field microscopic imaging; phase sensitive surface plasmon resonance (pSPR) bio-sensing technology based on differential phase measurement between radially polarized (RP) and azimuthally polarized (AP) beams to obtain an ultra-high sensitivity and a wide dynamic range simultaneously; the coupling between the localized surface plasmon (LSP) of metallic nano-particles and SPP virtual probe with longitudinal electric field to significantly improve the sensitivity of SERS system. With the integration of these three technologies in a single microscopic configuration, the system can achieve wide-field super-resolved imaging of biological specimens, ultra-high sensitivity for molecule detection and real-time monitoring for reaction process of biological samples simultaneously, fulfilling the requirement of multi-parameter multi-function real-time in-situ measurement of biological samples. The new microscopic scheme has great importance in real-time dynamic study on nano-scale biological living cells as well as accurate near field mapping.

In the past few years, silicon nitride planar waveguides have become a reference platform for nonlinear
nanophotonics, especially for Kerr frequency comb generation but also for spectral broadening and supercontinuum generation. In this work, we present several spectral broadenings using waveguides with different group velocity dispersion, some of them reaching a full octave span. By means of an innovative hyperspectral near-field microscopy technique, we fully characterize the spectral change that occurs during the propagation of light in the waveguide.
Optical near-field microscopy allows the mapping of the electromagnetic field with a resolution down to a few tens of nanometers, below the diffraction limit. Such a resolution is achieved by collecting the evanescent and propagative
fields using a dielectric probe made out of a tapered optical fiber whose extremity has a 50-nm diameter. While this technique has traditionally been used in linear optics with only one or a few wavelengths, it has recently been
extended to the mapping of the optical spectrum using a spectrometer and a fast camera. In this work, we use both a visible CCD camera and an InGaAs camera for infrared measurements around the pump wavelength at 1550 nm. The pump laser is a 100-fs pulsed laser source with a peak power of about 30 kW.
An hyperspectral measurement consists in recording the optical intensity for each position (x,y) on the sample and for each wavelength: a 3D matrix P(x,y,\lambda) is therefore obtained. From this raw data, several representations can be made. The spectra can be compared from point to point, when following the waveguide, allowing to better understand the nonlinear processes at stake during the supercontinuum generation. In particular, we show that depending on the width of the waveguide, the spectral broadening is qualitatively different owing to the
different regimes of dispersion. Our setup allowed us to measure the spectrum evolution on 1 cm of propagation, leading to an octave-spanning spectrum in the case of an anomalous-dispersion waveguide. In this case, spectral feature such as dispersive waves, third-harmonic generation and self-phase modulation give very clear and obvious signatures.
Hyperspectral near-field imaging also allows to image the multi-mode propagation within larger waveguides. In the case of spectral broadening in such waveguides, different spatial modes participate to the propagation and are differently visible depending on the wavelength. Clear interference patterns can be visualized using false-color imaging representations. This technique would provide much needed characterization for the emerging nonlinear optics in multimode waveguides research area.

Transformation optics is a general design tool that is as intuitive as the ray optics, but exact at the level of Maxwell’s equations. It provides a direct link between a desired electromagnetic phenomenon and the material response required for its occurrence. In this talk, I will give an overview of some recent progress of transformation optics, with a special focus on its applications at the subwavelength. 1. I will show how to use this strategy to design a finite subwavelength particle that can harvest light over a broadband spectrum like an infinite plasmonic system; 2. I will discuss how the conformal transformation approach can help us understand some complex plasmonic phenomena, such as fast electron interaction with singular plasmonic particles, van der Waals interaction at extreme scales, compact dimensions in 2D singular plasmonic surfaces, etc.

A three dimensional rigid spherical microscopic object can rotate in either the pitch, yaw or roll fashion. Among these, yaw motion has been conventionally studied using the intensity of the scattered light from birefringent microspheres through crossed polarizers. So far, however, there is no way to study the pitch rotational motion in spherical microspheres. Here we suggest a new method towards the study of such pitch motion in birefringent microspheres under crossed polarizers by measuring the 2-fold asymmetry in the scattered signal using video microscopy. We show a simple example of pitch rotation determination using video microscopy for a microsphere attached with a kinesin molecule while moving along a microtubule. It can also be extended to optical tweezers.

We propose and demonstrate experimentally an optical analogue of the famous Archimedes' screw where airborne particles are conveyed down or upstream the photons momentum ow through the rotation of a helical optical beam. We also report on the action of such a rotating screw on low-absorbing particles in a solution.

Dielectric nanoparticles, and silicon nanoparticles in particular [1,2], are becoming increasingly promising for various applications in photonics, nonlinear optics, optomechanics, and medicine. Plenty of applications exploit the benefits of low-loss Mie resonances, exhibited in the optical range by silicon nanoparticles with sizes of the order of 200 nm. The frequencies of the resonant Mie modes are determined by the size and shape of the particles. However, many of the fabrication techniques result in a polydisperse mixture of different sizes and shapes, and prompt for a post-processing to provide a uniform output. Having an entirely optical tool for such separation [3,4] is highly desirable for sterile, hazardous or highly dynamic microfluidic environments.
Following our recent publication [4], in this contribution we present the calculated optical forces acting on silicon nanoparticles in aqueous environment, analyse their potential for optical sorting in a number of schemes, and discuss the experimental implementation of the proposed methods in our setup.
The optical forces acting on silicon nanoparticles are shown to reveal their substantial dependence on the particle size. This dependence results in different velocities of the light-driven drift of the nanoparticles, depending on their size and the frequency of the incident light. We propose to employ these features to realise optical sorting, according to the following scenarios. First, we use two counter-propagating beams of different wavelength, which move particles of different sizes in opposite directions; by varying the intensity ratio between the two beams, different subsets of the particle sizes can be separated. A similar approach has been implemented for plasmonic particles [3]. Second, we suggest to impose two counter-propagating beams upon a uniform flow of a disperse mixture, which results in the particles of different sizes being pushed along different directions in space, so that an efficient angular separation is possible within certain size ranges. Third, we propose an efficient angular separation in an all-optical way, by directing the two beams at an angle. This scenario offers an efficient angular separation without any imposed flow.
In this work, we consider two laser beams with wavelengths of 532 and 638 nm. For this particular case, angular sorting scheme provides a unique size-angle dependence, yielding up to 70° span of deflections, in the size ranges of 120–160 nm, 190–220 nm, and a few smaller sets. We demonstrate that the proposed angular sorting techniques are robust against the Brownian motion, requiring a run of about 100 μm to achieve a 10-nm distinction in size, while using moderate (0.1 W) power. Finally, we consider the forces acting on silicon nanoparticles in the evanescent wave illumination and show that the proposed methods can be applied for a broad size dimensions using p-polarised light.
[1] Evlyukhin A.B., et al. Nano Lett. 12, 3749–3755 (2012).
[2] Kuznetsov A.I., et al. Sci. Rep. 2, 492 (2012).
[3] Ploschner M., et al. Nano Lett. 12, 1923–1927 (2012).
[4] Shilkin D., et al. ACS Photonics 4, 2312–2319 (2017).

The negatively charged nitrogen vacancy centre in diamond is known for its coherent spin properties and optical interface, and thus is regarded a promising candidate for quantum information applications [1]. Realisation of an efficient spin-photon interface with the NV centre is made challenging however by the fact that, in bulk diamond, only 3-4% of spontaneously emitted photons occur in the zero phonon line (ZPL). Placing NV centre in an optical cavity is being explored by several groups [2][3][4] as an effective way to selectively enhance the coherent emission of NVs and thereby increase the efficiency of the coherent spin-photon coupling. Previous work reported successful coupling of the NV in nano-diamond to an open access micro-cavity and observed enhanced ZPL emission [5]. However the NV centres in nano-diamond suffer from broadened zero phonon transition and poor spin coherence. By fabricating NV centres in a ~micrometre thick membrane of high purity single crystal material we can take advantage of the tunability of open access cavities, and at the same time, provide close-to-bulk crystal environment to maintain the coherent spin properties of the NV centres. Here we report our work on the tunable cavity coupling of the ZPL of a NV centre in a 1.2micrometre-thick diamond membrane at 4K. The diamond membrane is fabricated from a 0.5mm-thick E6 CVD diamond plate where ion implantation is carried out on both surfaces to create NV centres at the depth of around 70nm. The plate is then machined into 30micrometre-thick slices, and thinned by ICP-RIE with a combination of Ar/Cl[6] and pure oxygen plasma etching recipes. The open cavity consists of a concave mirror (99.99% reflectivity) deposited on a template fabricated using Focused Ion Beam (FIB) milling[7] and a planar mirror (99.8% reflectivity) which supports the membrane. For bare cavities with mirror radii of curvature (RoC) of 12micrometre, we measured a finesse of F~2000 and mode volume as small as 0.75micrometre^3. In-situ tuning of the cavity resonance is achieved with piezoelectric actuators. When mounted in our bath cryostat the cavity modes have dominant Lorentzian line profiles which indicate a passive stability of the cavity length of better than 0.15nm. No active locking is currently deployed. With the presence of a diamond membrane inside the cavities, the measured finesse and mode volume of a cavity with 12micrometre RoC are found to be around 300 and 3 micrometre^3, respectively. We attribute the reduction in finesse to scattering at the membrane-air and membrane-mirror interfaces. On coupling to the ZPL of a target NV centre, we record a factor of 4 increase in the saturated intensity of ZPL fluorescence compared to that measured from the same NV centre in absence of the concave mirror. This result is consistent with the calculated Purcell factor of 16 combined with a relatively low efficiency of light extraction (estimated to be around 19%) from the cavity due to the scattering losses.

We report on the optical properties of recently developed telecom-wavelength quantum dots based on the GaAs material system. In order to achieve the InAs quantum dot wavelength shift towards the telecom C-band, strain-relaxation with the help of an InGaAs metamorphic buffer is realized. The general emission properties of the quantum dot ensemble and single dots is analyzed, containing analysis of lifetime measurements and fine-structure splitting investigations. Single-photon emission is verified for excitation in continuous wave and pulsed mode. The generation of polarization-entangled photon pairs via the biexciton-exciton radiative cascade is shown, even for increasing time windows and time delays.

Metamaterials are artificial subwavelength structures. By altering energy-momentum dispersion, metamaterial allows a control of photophysical property of emitters located nearby. Furthermore, the presence of plasmonic layers in composition of metamaterial permits an image-dipole interaction. It will be examined how a series of photophysical properties including charge transfer in donor-acceptor structure and spectral shift of intramolecular charge transfer emitters are modified nearby metamaterials.

Polarized time resolved fluorescence measurements are used to characterise the structure of the two-photon tensor in the enhanced green fluorescent protein (EGFP) and predict the “hidden” degree of hexadecapole transition dipole alignment 〈α40〉 created by two-photon absorption (TPA). We employ a new method for the accurate STED measurement of the evolution of 〈α40〉 by analysing the saturation dynamics of the orthogonally polarized components of two-photon excited EGFP fluorescence as a function of the time delay between the 800 nm pump and 570 nm dump pulses. The relaxation of 〈α40〉 by homo-FRET is found to be considerably greater than that for the fluorescence anisotropy which directly measures the quadrupolar transition dipole moment alignment 〈α20〉. Our results indicate that higher order dipole moment correlation measurements promise to be a sensitive probe of resonance energy transfer dynamics.

Semiconductor quantum dots (QDs) are a promising “nano-antennas” capable of absorbing efficiently light energy upon one- or two-photon excitation and then transferring it to convenient energy acceptors via Förster resonance energy transfer (FRET). The photosensitive protein bacteriorhodopsin (bR) has been shown to be a promising material for optoelectronic and photovoltaic applications, but it cannot effectively absorb light in the UV, blue, and NIR regions. It was shown previously that formation of hybrid complexes of QDs and purple membranes (PMs) containing bR could significantly improve the bR capacity for utilizing light upon one- and two-photon laser excitations. Under the laser irradiation, the optical properties of bR itself remain unchanged, whereas those of QDs may be altered. Therefore, it is important to study the effects of intense laser excitation on the properties of the QD–PM hybrid material. In this study we have shown that laser irradiation can lead to an increase in the luminescence quantum yield (QY) of QDs. The fact that this irradiation does not change the QD absorption spectra means that the QD quantum yield may be optically controlled without changing the QD structure or composition. Finally, we have shown experimentally that photoinduced increase in the QY of QDs lead to the corresponding increase in the efficiency of FRET in the QD–PM hybrid material. As a result, an approach to increasing the FRET efficiency in hybrid nano-biomaterials where QDs serve as donors have been proposed.

The issue of whether the optical orbital angular momentum of light can play any significant role in chiroptical interactions has seen a resurgence of interest in the past few years. Revising preliminary expectations, it has been shown both theoretically and experimentally that the topological charge can indeed play a decisive role in some chiroptical interactions, with the rates of these optical phenomena proving sensitive to the sign of the vortex charge &ell;. Using quantum electrodynamics, it is now revealed how the inclusion of molecular electric-quadrupole transition moments in both chiral and achiral anisotropic media produces such an effect. Specifically, for single-photon absorption it transpires that both the orbital and spin angular momentum must be engaged through a circularly polarized vortex beam. The chiroptical effect is identified as a manifestation spin-orbit interaction in light.

The capacity to tailor as wanted the fluorescence’s properties of a fluorophore increases the number of applications were the same fluorophore can be useful, like in imageology. One way to modify these properties is the presence of plasmonic fields nearby the fluorophore, and their origin can be the surface plasmons generated in metallic nanoparticles, like silver and gold, when these are excited. Usually fluorescence quantum yield is studied by conventional fluorescence spectroscopy techniques, but these are subjected to errors from reflection or refraction from the sample and a way to avoid these errors is to use indirect measurements techniques as in the case of thermal lens spectroscopy, which measures the change generated by the sample’s absorption of radiation, instead of measuring the absorption per se as regular spectroscopic methods. This technique is based in the photoinduced refraction index’s change. In this work we studied the effect that silver nanoparticles had in the fluorescence’s properties of ethanolic solutions of rhodamine B, specially its quantum yield, using a mode-mismatched thermal lens setup. We found that the presence of silver nanoparticles lowers the dye’s quantum yield between 4% and 38% which depends on the dye and nanoparticles’ concentrations. The thermal diffusivity’s values showed that the silver nanoparticles are increasing the non-radiant decay velocity of the rhodamine b, which is the reason why the quantum yield gets lower. These results not only gave us information about the studied samples, but also validate the capacity of a mode-mismatched thermal lens system to study fluorescence properties.

The molecular clusters, so called J-aggregates of pseudoisocyanine dye, were obtained in ordered cylindrical nanopores of anodic aluminum oxide. The absorption and luminescence of the samples were studied by the VIS-spectroscopy and laser confocal microscopy. The band of J-aggregates has the same shape, but is inhomogeneous broadened in comparison with solution. The luminescence maximum of J-aggregates was observed at 578 nm upon excitation at 543 nm as well as at 405 nm. Non-resonant luminescence excitation occurred due to energy transfer from oxygen vacancy of alumina to molecular nanoclusters. This is also confirmed by time-resolved luminescence spectroscopy, which shows the increase of luminescence decay time of J-aggregates placed in alumina up to the luminescence time of the clean alumina in comparison with J-aggregates coated on glass substrate.

As a class of semiconductors, transition metal dichalcogenides (TMDs) have the formula MX2, where M stands for a transition metal (i.e., Mo, W, Ti, Nb, etc.) and X stands for a chalcogen (i.e., S, Se or Te). TMDs show graphene-like layered structure. Strong covalent bonds in layers and weak van der Waals interaction between layers allow TMDs to form a robust 2D nanostructure. In a TMD monolayer, the single transition metal layer is sandwiched between the two chalcogen layers. Owing to the specific 2D confinement of electron motion and the absence of interlayer coupling perturbation, 2D layered TMDs show unique photonics-related physical properties, e.g.,
1) Sizable and layer-dependent bandgap, typically in the 1-2 eV range;
2) Indirect-to-direct bandgap transition as the decreasing of the number of monolayer;
3) Fairly good photoluminescence and electroluminescence properties;
4) Remarkable excitonic effects, i.e., high binding energy, large oscillator strength and long lifetime.
In combination of the ultrafast carrier dynamics and molecular-scale thickness, the prominent properties manifest the 2D TMDs a huge potential in the development of photonic devices and components with high performance and unique functions.
We have extensively studied the ultrafast nonlinear absorption and nonlinear refraction of layered MX2 (X=S, Se, Te) over broad wavelength (Vis-NIR) and time (fs-ps-ns) ranges. Large area MoS2 neat films with controllable thicknesses were fabricated from liquid-exfoliated MoS2 dispersions by vacuum filtration. The MoS2 films show superior broadband ultrafast saturable absorption (SA) performance, in comparison with the graphene films and the MoS2 dispersions. Very recently, we observed giant two-photon absorption (TPA) coefficient in a WS2 monolayer. The order of magnitude of TPA coefficient in WS2 monolayer (~100 cm/MW) exceeds that of the conventional semiconductors (e.g., CdTe, GaAs, ZnS, ZnO, etc.) by a factor of 3-4. This is also the first Z-scan performance on an optical medium with a thickness as tiny as 0.75 nm. Moreover, a comprehensive study on the layer-dependent nonlinear photonic effect was carried out in MoS2 mono- and few-layers by CVD growth. SA to TPA transition was confirmed when the thickness changes from few-layer to monolayer. In addition, a spatial self-phase modulation method has been applied to tune the nonlinear refractive index of TMD dispersions.
The above-mentioned works have opened up a door towards 2D semiconductor based nonlinear photonics, spectroscopy and relevant photonic devices.

High-harmonic generation (HHG) in condensed-matter systems is both a source of fundamental insight into quantum electron motion and a promising candidate to realize compact ultraviolet and ultrafast light sources [1-3]. Here we argue that the large light intensity required for this phenomenon to occur can be reached by exploiting localized plasmons in conducting nanostructures. In particular, we demonstrate that doped graphene nanostructures combine a strong plasmonic near-field enhancement and a pronounced intrinsic nonlinearity that result in efficient broadband HHG within a single material platform [4]. We extract this conclusion from time-domain simulations using two complementary nonperturbative approaches based on atomistic one-electron density matrix and massless Dirac-fermion Bloch-equation pictures, where the latter treatment is supplemented by a classical electromagnetic description of the self-consistent field produced by the illuminated nanostructure. High harmonics are predicted to be emitted with unprecedentedly large intensity by tuning the incident light to the localized plasmons of ribbons and finite islands. In contrast to atomic systems, we observe no cutoff in harmonic order, while a comparison of the predicted HHG from graphene to that observed in solid-state systems suggests that the HHG yields measured in semiconductors can be produced by graphene plasmons using 3-4 orders of magnitude lower pulse fluence. Our results support the strong potential of nanostructured graphene as a robust, electrically-tunable platform for HHG.
[1] S. Ghimire et al., “Observation of High-Order Harmonic Generation in a Bulk Crystal,” Nat. Phys. 7, 138 (2011).
[2] O. Schubert et al., “Sub-Cycle Control of Terahertz High-Harmonic Generation by Dynamical Bloch Oscillations,” Nat. Photon. 8, 119 (2014).
[3] T. T. Luu et al., “Extreme Ultraviolet High-Harmonic Spectroscopy of Solids,” Nature 521, 498 (2015).
[4] J. D. Cox, A. Marini, and F. J. García de Abajo, “Plasmon-Assisted High-Harmonic Generation in Graphene,” Nat. Commun. 8, 14380 (2017).

Chirality is a general phenomenon in nature. Many biomolecules in our body such as DNA and enzymes are chiral. The enantiomers existing in oranges and lemons cause different smells. More importantly, while one chirality forms a powerful medication, the other may cause very serious side effects, for instance in the case of left- and right-handed Thalidomide. It is hence of crucial significance to understand chirality for the purpose of interpreting chirality in biology as well as employing chirality for sensing applications in chemistry, pharmacy, etc.
Chiral plasmonics holds great potential in the sense that it has a large range of flexibility to mimic natural chiral substances and simultaneously exhibits a giant chiroptical response arising from the strongly confined and enhanced electro-magnetic field. Nonlinear chiral plasmonics is even more desired since the nonlinear chiroptical effects might be orders of magnitude higher than their linear counterparts. Until now, both linear and nonlinear chiroptical properties in tailored chiral plasmonic systems have been investigated. However, the underlying physical mechanism for nonlinear plasmonic chirality is far from being understood and further quantitative modelling is particularly missing.
Here we study the third-order chiroptical responses of a 3D chiral structure consisting of identical corner-stacked gold nanorods, the so-called plasmonic Born-Kuhn analog. The structures were fabricated by a multi-layer electron-beam lithography (EBL) technique. First, the glass substrate was covered by a dielectric spacer layer (IC1-200, Futurex) via spin-coating. Second, EBL processing procedures (electron-beam exposure of a PMMA resist, development, evaporation of a gold film, and subsequent lift-off) were implemented to fabricate one layer of gold nanorods. The rod lengths were varied to tune the plasmonic resonances in the range of 920-1150 nm, which match the spectral window of our ultrafast laser source. Third, another IC1-200 spacer layer was spin-coated above the layer of gold nanoantennas. Fourth, employing a second EBL cycle assisted by precise positioning, the second layer of gold nanorods was finished. Finally, a third IC1-200 spacer layer was planarized on top in order to create isotropic environment for the gold nanostructures. The thickness of the IC1-200 spacer layer was selected to ensure strong coupling via optical near-field between the two layers of gold nanorods. A C4 geometrical symmetry was designed to eliminate linear birefringence in the structures.
A home-made wavelength-tunable laser source with 60 fs pulse duration was used as the pump for nonlinear frequency conversion. Circularly polarized fundamental light was realized by combination of a polarizer and a broadband quarter waveplate. The third-harmonic-generation signals were recorded by a CCD camera attached to a spectrometer. Nonlinear chiroptical spectroscopy was performed by tuning the wavelength and switching the handedness of the fundamental light.
To interpret the nonlinear chiroptical responses, we utilized a coupled anharmonic oscillator model, in which the coupling term of the two layers of gold nanorods and the phase retardation of the incoming fundamental and outgoing generated wave are fully considered. In this way, we achieve good agreement between experimental measurement and analytical prediction. This quantitative model addresses the origin of the nonlinear chiroptical effects and is very instructive for the efficient design of plasmonic chiral structures for giant nonlinear circular dichroism. Our research extends the present understanding of chiral plasmonic systems and paves the way towards ultrasensitive nonlinear chiral sensing.

Saturable absorption (SA) is an extreme nonlinear phenomenon that consists of the quenching of optical absorption under high-intensity illumination. This effect, which is an inherent property of photonic materials, constitutes a key element for passive mode-locking (PML) in laser cavities, where continuous waves are broken into a train of ultrashort optical pulses. Most materials undergo SA at very high optical intensities, in close proximity to their optical damage threshold. Currently, state-of-the-art semiconductor-based SA mirrors are routinely employed for PML lasers. However, these mirrors operate in a narrow spectral range, are poorly tunable, and require advanced fabrication techniques. Recently, carbon nanomaterials have emerged as an attractive, viable, and cost-effective alternative for the development of next-generation PML lasers. For example, carbon nanotubes undergo SA at rather modest light intensities, while their operation wavelength (determined by the energy band gap) can be manipulated by tuning their diameter. Broadband operation has been demonstrated by using an ensemble of CNTs with a wide distribution in diameter, at the expense of higher linear loss from off-resonance tubes. Graphene overcomes this limitation thanks to its peculiar conical band structure, which gives rise to broadband resonant SA at remarkably low light intensity that can further be tuned by means of an externally applied gate voltage. Graphene-based SA components have been used to achieve PML ultrafast laser operation, broadband tunability, and quality-factor switching. Graphene multilayers have also been employed to generate large energy pulses and to achieve PML in fiber lasers with normal dispersion. In addition, recent theoretical investigations predict single-mode operation of random lasers by embedding graphene flakes in a gain medium.
Here we calculate intraband and interband contributions to SA of extended graphene by nonperturbatively and semianalytically solving the single-particle Dirac equation for massless Dirac fermions (MDFs) in the presence of an external electromagnetic field retaining only one-photon processes. We further investigate the dependence of the intensity-saturated grapheme conductivity on doping, temperature, and optical frequency. Interestingly, we find a remarkably low intensity threshold for SA, which is consistent with available experimental reports. Our calculations indicate a strong quenching of absorption depth produced by electrical doping (which can be controlled through gating), as well as a weak dependence on electron temperature. Additionally, through time-domain simulations based on an atomistic tight-binding/single-particle density-matrix formalism, we study SA in graphene nanoribbons, including finite-size effects and electron-electron interactions that play a significant role in the optical response of nanostructured graphene. Surprisingly, we find that while the linear absorption predicted in atomistic simulations is reduced compared to that of extended graphene, its nonlinear saturation intensity threshold is in good quantitative agreement with predictions based on the MDF model. Deviations from the semianalytical treatment occur only at high doping, where SA is quenched and multiphoton processes lead to an intensity-dependent increase of absorption. We anticipate that the present findings will impact the future development of graphene-based PML fibre lasers and single-mode random lasers.

We propose and demonstrate photonic crystals (PCs) providing backward-directional propagation of surface slow waves, which is significant for potential PC-based photonic applications. An effective pathway for backward directing of surface slow light along with the modification of other important characteristics is presented via implementing surface morphological diversity in PCs. With the surface orientation angle varying from 900 to 300, the newly appearing bands inside the band gap shift to higher frequencies, and negative group indices up to -100 are observed as the strong indication of backward propagation. Furthermore, dependence of the propagation direction on the surface corrugation angle has been verified via detailed time-domain analyses and microwave experiments using dipole source. As obtained from both the numerical and experimental results, for instance, the structure with 600 provides a well-defined backward propagation. In addition, normalized-delay-bandwidth-product can easily be modified by varying the surface orientation angle in the proposed structures according to the necessities of the application. Furthermore, the group velocity dispersion spectra extracted for each periodic structure exhibit considerably high-range near-zero values as 0.139 ps2/m at 900 for the range of 495.54-501.25 nm and 0.176 ps2/m at 85° for the range of 495.66-501.25 nm. Third order dispersion spectra also obtained for the proposed PCs show near-zero values as 0.098 ps3/m at 900 and 0.113 ps3/m at 85° in the corresponding frequency regimes. Facile control of the key characteristics such as backward-directed surface wave propagation in the periodic dielectric structures having morphological diversity serves a great potential for nextgeneration photonic applications.

Orbital angular momentum is a fundamental degree of freedom of light that manifests itself even at the single photon level. The coherent generation and beaming of structured light usually requires bulky and slow components. Using wave singularities known as bound states in continuum, we report an integrated device that simultaneously generates and beams powerful coherent beams carrying orbital angular momentum. The device brings unprecedented opportunities in the manipulation of micro-particles and micro-organisms, and, will also find applications in areas such as biological sensing, microscopy, astronomy, and, high-capacity communications.

Josephson dynamics, spontaneous symmetry breaking and quantum criticality are fascinating physical phenomena that can be realized today in coupled dissipative optical cavities with nonlinear interactions. Among the different experimental test-beds, photonic crystal coupled nanocavities operating in the laser regime are outstanding systems since nonlinearity, gain/dissipation and intercavity coupling can be judiciously tailored [1].
Complex photon statistics is inherent to the nature of nanolasers due to the presence of strong spontaneous emission noise. Yet, although most common scenarios emerge from quasi-dynamical equilibrium where the gain nearly compensates for losses, little is known about far-from-equilibrium statistics resulting, for instance, from a rapid variation of a parameter or "quench".
Our nanolasers are fabricated in suspended 2D InP-based Photonic Crystal membranes, and studied as a function of pump power and coupling strength. The modification of coupling strength is obtained by an original engineering procedure that allows us to tune the coupling strength between the nanocavities without affecting the nanolaser performance [1].
Under short (100 ps) pulse pumping, the strongly coupled laser nanocavity system exhibits two modes: a strong lasing mode, which has an anti-symmetric energy distribution, and a weak nonlasing one, possessing a symmetric energy distribution. We implement a simple experimental technique –single pulse energy detection scheme– that allows us to measure the statistical distributions of the photon number of both modes simultaneously. In particular, we analyze the photon number distributions of the weak one and link, using a mean field model, both the emergence of fat tails in the distributions and the superthermal nature of the emission through second order correlation (g2) measurements. We conclude that transient dynamics after quench, when projected onto the nonlasing mode, generically exhibit long-tailed superthermal light.
Such an optical quench mechanism is akin to the fast cooling of a suspension of Brownian particles under gravity, with the inverse temperature of the reservoir playing the role of the intracavity intensity. We show that passing through the lasing threshold corresponds to an abrupt decrease of the contribution of spontaneous emission —that plays the role of an effective temperature— during which the statistics of the nanolaser trajectories in phase space are dominated by nonlinear transport.
Probability density functions enabled the experimental quantification of the distance from thermal equilibrium –and hence the degree of residual order– via the thermodynamic entropy. This allowed us to further detect mixing of thermal states and coherent broken parity phases, which are beyond the simple Brownian particle description [3].
REFERENCES
1. Hamel, P., et al., “Spontaneous mirror-symmetry breaking in coupled PhC nanolasers,” Nat. Phot., Vol. 9, 2015.
2. Marconi, M., et al., “Asymmetric mode scattering in strongly coupled photonic crystal nanolasers,” Optics Letters, Vol. 41, 5628, 2016.
3. Marconi, M., et al, “Quenched phases in strongly coupled dissipative optical cavities. ” arXiv preprint arXiv:1706.02993.

Nanophotonic components operate in the few-photon regime, thus their experimental characterization calls for photon-counting techniques, at least in the threshold region, and requires adequate laser models to interpret the observations.
While the photon statistics of (macroscopic) Class A [1] lasers is well understood and can be readily reconstructed from the zero-delay second order autocorrelation (g(2)(0)), the memory effects introduced by the slow material response of semiconductor-based devices (Class B [1]) and the sensitivity of nanolasers to spontaneous emission [2] require a more careful approach. The latter induces a spontaneous spiking dynamics [3], near threshold, resulting in values of g(2) larger than those expected even for a chaotic signal, and growing without bounds as the duration of the spikes decreases.
Currently available laser models appear unable to predict such a behaviour, due to an inadequate treatment of the contribution of spontaneous emission, and, since the Probability Density Function (PDF) collects into a statistical distribution the state of the system, its predictions fail when the dynamics is not reproduced by the model from which it is derived. Thus, contrary to the usual assumption, the validity of the photon statistics of macroscopic Class B devices [4] must be reconsidered as the cavity volume is reduced.
We investigate the influence of the cavity size on a commercial VCSEL microlaser with a moderate fraction of spontaneous emission coupled into the lasing mode (beta~0.0001), which represents a happy compromise between a large enough cavity size to detect the dynamics while capturing the self-spiking typical of very small lasers. The autocorrelation (g(2)(0)) is both computed from the intensity time series with a fast (10 GHz) photodetector and deduced from the measured coincidences in arrival times of a photon counting apparatus (TAC with 15 ps timing resolution) in Hanbury-Brown & Twiss (HBT) configuration. We observe values of g(2)(0) up to 2.2, which would produce exponentially decaying distributions if interpreted through the current models [4]. Instead, the experimental PDF, reconstructed from the time series, matches the generic distributions for class B lasers even for the maximum value of g(2)(0).
We therefore conclude that these two techniques cannot be considered as providing equivalent information if only the second order moment g(2) of the distribution is considered, and that new theoretical work is needed on the photon statistics of small-sized Class-B lasers.
References
[1] J. R. Tredicce, F. T. Arecchi, G. L. Lippi, and G. P. Puccioni, J. Opt. Soc. Am B2, 1, 173-183, 1985.
[2] G. P. Puccioni and G. L. Lippi, Opt. Express 23, 3, 2369-2374, 2015.
[3] T. Wang, G.P. Puccioni, and G.L. Lippi, Sci. Rep. 5, 15858 (2015).
[4] P. Paoli, A. Politi, and F.T. Arrecchi, Z. Physik B 71, 403-410, 1988.

In this work, we perform numerical studies of two photonic crystal membrane microcavities, a short line-defect L5 cavity with relatively low quality (Q) factor and a longer L9 cavity with high Q. We compute the cavity Q factor and the resonance wavelength λ of the fundamental M1 mode in the two structures using five state-of- the-art computational methods. We study the convergence and the associated numerical uncertainty of Q and λ with respect to the relevant computational parameters for each method. Convergence is not obtained for all the methods, indicating that some are more suitable than others for analyzing photonic crystal line defect cavities.

In this paper, we study the use of Titanium Nitride (TiN) as a new alternative plasmonic material to achieve a highly sensitive surface plasmon resonance (SPR) photonic crystal fiber (PCF) biosensor. The TiN has unique properties that make it an ideal material for nanofabrication, where TiN is highly stable, highly conductive, and corrosion resistant. Full vectorial finite element method is used with perfectly matched layer (PML) as boundary conditions to analyze the suggested biosensor. By analyzing the geometrical parameters of the proposed biosensor, a refractive index sensitivity of 7700 nm/RIU and 3600 nm/RIU are obtained for quasi-transverse electric (TE) and quasi transverse magnetic (TM) modes, respectively. The reported biosensor has a high linearity for detecting an unknown analyte refractive index ranging from 1.32 to 1.34. Further, fabrication of the proposed biosensor could be done using standard PCF fabrication current technologies.

We propose a novel approach in optical trapping exploiting mesoscopic photonic crystal microcavities. Full light confinement in mesoscopic photonic crystal membranes, forming a mesoscopic self-collimating 1D Fabry-Pérot cavity, was theoretically predicted and experimentally verified by the authors in previous papers. In this paper, we numerically demonstrate a high performance MPhC microcavity for optical trapping of fine particulate matter in air. The MPhC cavity has been simulated by 3D FDTD simulations while the trapping potential has been evaluated by means of the gradient force density convolution method. We numerically show that it is possible to obtain very high trapping potential for polystyrene particles having radii as small as 245 nm.

Plasmonic band gap is a range of frequencies, within which, surface plasmon polaritons cannot propagate for any wavevector. Unfortunately the first plasmonic band gap cannot be observed directly in reflectance spectroscopy [1]. To detect it, biharmonic metal-air surface structuring is conventionally utilized [2,3]. However in this case experimental geometry is strictly limited to normal angle of incidence, which is not compatible with large range of applications.
In current work we introduce biperiodic plasmonic crystals. We experimentally demonstrate, that biperiodic structuring allows to tune band gap spectral-angular position.
Laser interference lithography (LIL) is a well-established technique for creating periodic planar nanostructures over a large surface area. LIL allows to precisely control the modulation period and depth and thus perfectly match diffraction coupling conditions and tune plasmonic band gap properties.
We used LIL experimental setup based on Lloyd interferometer. The radiation from the laser source (He-Cd, wavelength 325 nm, average power 14 mW) was spatially filtered and then formed interference pattern on the silicon wafer, covered with a thin layer of SU-8 2015. The structure period was defined by the incident angle on the interferometer. Modulation depth was defined by exposure time. By applying subsequent second exposure with another angle of incidence, we obtained biperiodic structure. Exposed samples were washed in corresponding developer, dried in air and later sputtered with 100 nm of aluminium.
We fabricated a set of biperiodic plasmonic crystals with different periods and modulation depths. The quality and geometrical parameters of biperiodic plasmonic crystals were monitored by scanning electron microscopy and atomic force microscopy. The appearance of plasmonic band gap was measured by spectral-angular polarisation spectroscopy. We experimentally determined the dependance of plasmonic band gap properties (width and position) on geometrical parameters of biperiodic plasmonic crystals. We also performed FDTD numerical simulations (Lumerical). The experimental results are in good agreement with numerical calculations.
[1] Raether, Heinz. [Surface Plasmons on Smooth and Rough Surfaces and on Gratings.], Springer Berlin Heidelberg, 91-105 (1988).
[2] Barnes, William L., et al. "Physical origin of photonic energy gaps in the propagation of surface plasmons on gratings." Physical Review B 54.9 (1996): 6227.
[3] Kocabas, Askin, S. Seckin Senlik, and Atilla Aydinli. "Plasmonic band gap cavities on biharmonic gratings." Physical Review B 77.19 (2008): 195130.

Among the quantum systems capable of emitting single photons, the class of recently discovered defects in hexagonal boron nitride (hBN) is especially interesting, as these defects offer much desired characteristics such as narrow emission lines and photostability. Like for any new class of quantum emitters, the first challenges to solve are the understanding of their photophysics as well as to find ways to facilitate integration in photonics structures. Here, we will show our investigation of the optical transition in hBN with different methods: Employing excitation with a short laser pulse the emission properties in case of linear and non-linear excitation can be compared [1]. The possibility to perform two-photon excitation makes this single photon emitter an interesting candidate as a biosensor. We further show the behaviour of defects in hBN when being excited with different wavelengths and deduce the consequences for its level scheme. Here, it is found that the quantum efficiency of the emitters varies strongly with excitation wavelength, a strong indication of a branched level system with different decay pathways.
[1] A W Schell et al., APL Photonics 1, 091302 (2016)
[2] A W Schell et al., arXiv:1706.08303 (2017)

Metasurfaces provide great feasibilities for tailoring both propagation waves and surface plasmon polaritons (SPPs). Manipulation of SPPs with arbitrary complex field distribution is an important issue in integrated nanophotonics due to their capability of guiding waves with subwavelength footprint. Here, with metasurface composed of nano aperture arrays, a novel approach is proposed and experimentally demonstrated which can effectively manipulate complex amplitude of SPPs in the near-field regime. Positioning the azimuthal angles of nano aperture arrays and simultaneously tuning their geometric parameters, the phase and amplitude are controlled based on Pancharatnam-Berry phases and their individual transmission coefficients. For the verification of the proposed design, Airy plasmons and axisymmetric Airy beams are generated. The results of numerical simulations and near-field imaging are well consistent with each other. Besides, 2D dipole analysis is also applied for efficient simulations. This strategy of complex amplitude manipulation with metasurface can be used for potential applications in plasmonic beam shaping, integrated optoelectronic systems and surface wave holography.

A modified nanocone nanowire (NW) is proposed and analyzed for solar cell applications. The suggested NW consists of conical and truncated conical units. The geometrical parameters are studied by using 3D finite difference time domain (FDTD) method to achieve broadband absorption through the reported design and maximize its ultimate efficiency. The analyzed parameters are absorption spectra, ultimate efficiency and short circuit current density. The numerical results prove that the proposed structure is superior compared to cone, truncated cone and cylindrical nanowires (NWs). The reported design achieves an ultimate efficiency of 44.21% with an enhancement of 40.66% relative to the conventional conical NWs. Further, short circuit current density of 36.17 mA/cm2 is achieved by the suggested NW. The modified nanocone has advantages of broadband absorption enhancement, low cost and fabrication feasibility.

The magnetooptical control of light implies different directions of polarization plane rotation, linear-to-circular and other polarization transformations. These opportunities can be opened using polarization-sensitive resonances, for example, Bloch surface waves (BSWs) and waveguide modes (WGMs) in magnetophotonic crystals (MPCs) [1]. Magneto-optical phenomena, such as Faraday effect, can be significantly increased near spectrally narrow optical resonances [2]. In this case, the Faraday rotation angle is determined both by the magnetic properties of the material and by the Q-factor of the resonances, which define their spectral width. The resonance of the BSW is shown to be extremely narrow. The proper choice of the MPC parameters gives the ways to observe the s-polarized BSW and p-polarized WGM of the MPC in the same spectral region. Here, we experimentally demonstrate how a fundamental property of magneto-optical effects to couple two linearly polarized modes allows one to control and modify the values of Faraday rotation angles. The interplay of the BSW and WGM results in an enhancement of the Faraday rotation angle, change of lineshape of Faraday rotation spectra and direction of the polarization rotation.
Reflectance spectra of the one-dimensional magnetophotonic crystal and the corresponding Faraday rotation spectra were experimentally measured using the Kretschmann attenuated total internal reflection configuration and numerically calculated using the transfer matrix approach [3]. The studied magnetophotonic crystal sample consists of 15 alternating layers of fused quartz and Bi-substituted yttrium-iron-garnet on a sGGG substrate.
The BSW excitation corresponds to a narrow resonance in the reflectance spectra of the s-polarized light. Wide dips in the reflectance spectra of p-polarized light correspond to the WGM resonances. As the incident angle increases, both the resonances shift to short wavelengths, but the WGM resonance shifts faster; thus, the spectral distance between the BSW and WGM resonances decreases. The spectral dependence of the Faraday rotation angle of s-polarized light has a feature coinciding in the BSW wavelength and caused by the BSW excitation. The feature in the Faraday rotation spectrum has a Fano resonance shape and changes from an asymmetric shape to a symmetric one, while the incident angle increases and the BSW and the WGM resonances approach each other. This behavior is observed both in the experiment and calculations. Thus, it can be argued that the spectral dependence of the Faraday rotation angle depends not only on the BSW resonance in the structure but also on the coupling of the BSW with the WGM mode that is not excited in the s-polarization of the incident light. Besides, the Faraday rotation changes direction while BSW and the WGM resonances spectral position approach each other that makes these resonance in MPCs promising for the future photonics devices.
[1] M. N. Romodina, I. V. Soboleva, A. I. Musorin, Y. Nakamura, M. Inoue, A. A.
[2] M. Inoue, M. Levy, A.V. Baryshev, Magnetophotonics: From Theory to Appli- cations, Springer Series in Materials Science, 2013.
[3] D.W. Berreman, J. Opt. Soc. Am. 62, 502–510 (1972).

There is currently significant interest in operating devices in the quantum regime, where their behaviour cannot be explained through classical mechanics. Quantum states, including entangled states, are fragile and easily disturbed by excessive thermal noise. Here we address the question of whether it is possible to create non-reciprocal devices that encourage the flow of thermal noise towards or away from a particular quantum device in a network. Our work makes use of the cascaded systems formalism to answer this question in the affirmative, showing how a three-port device can be used as an effective thermal transistor, and illustrates how this formalism maps onto an experimentally-realisable optomechanical system. Our results pave the way to more resilient quantum devices and to the use of thermal noise as a resource.

Strong coupling between excitons and light leads to the formation of hybrid states with mixed properties of light and matter. As a result, interesting physical phenomena have been observed at room temperature, e.g. Bose–Einstein condensation and superfluidity, and novel applications are emerging, such as low threshold lasers and quantum devices. Recently it was shown that metasurfaces of aluminum nanoantennas coated with molecular J-aggregates can provide an excellent platform for the formation of strongly coupled exciton-localized surface plasmons (X-LSPs). However, their optical nonlinearities and temporal dynamics are still not well understood. In this work, we use femtosecond pump-probe spectroscopy to study X-LSPs in such composite Al/molecular metasurfaces on time scales that are longer than their Rabi oscillation period. We study the linear and nonlinear optical properties of the uncoupled and hybrid systems and find that the nanoscale plasmonic confinement in metallic nanoparticle cavities introduces intriguing new ultrafast phenomena in the strong coupling regime. These include modifications of the hybrid system due to femtosecond changes in the molecular environment, picosecond oscillations due to acoustic breathing modes of the nanoantennas, and long relaxation times of the nonlinear perturbation at the upper X-LSPs frequency band.

Geometric Spin Hall effect of light (SHEL) has attracted considerable attention because of the universality of this variant of the spin Hall shift and its independence on the light-matter interactions and materials properties. However, the magnitude of geometric SHEL is typically very small, in the sub-wavelength domain, making it difficult for direct experimental observation. Here, we have applied weak measurement schemes to amplify and faithfully observe the geometric SHEL. In our experiment, the input beam is pre-selected in linear polarization basis (p- or slinear polarization) and post selections are done at nearly orthogonal linear polarizations (small angle ϵ away from the exact orthogonal).This results in weak value amplification (~cotϵ) of the resulting shift of the beam centroid. Moreover, this process also leads to selective conversion of spatial to angular nature of geometric SHEL, which along with the weak value amplification leads to manifold amplification of the resulting SHEL, enabling its reliable experimental detection. A key feature of our weak measurement scheme is that both the weak perturbation (tiny geometric SHEL) and the post selection are done simultaneously by a single optical element, namely, a linear polarizer. The dependence of the weak value of the geometric SHEL on the pre and the post selection of polarization states in both linear and elliptical basis were also investigated in details. The specifics of the different weak measurement schemes, various intriguing experimental manifestations of the weak value amplified geometric SHEL, their analysis / interpretation via polarization operator based treatment of weak measurements is presented here.

The short wavelength of graphene plasmons relative to the light wavelength makes them attractive for
applications in optoelectronics and sensing. However, this property limits their coupling to external
light and our ability to create and detect them. More efficient ways of generating plasmons are therefore
desirable. Here we demonstrate through realistic theoretical simulations that graphene plasmons can be
efficiently excited via electron tunneling in a sandwich structure formed by two graphene monolayers
separated by a few atomic layers of hBN. We predict plasmon generation rates of ~ 10^12 - 10^14 1/s over
an area of the squared plasmon wavelength for realistic values of the spacing and bias voltage,
while the yield (plasmons per tunneled electron) has unity order [1]. Our results support electrical
excitation of graphene plasmons in tunneling devices as a viable mechanism for the development of
optics-free ultrathin plasmonic devices.
[1] S. de Vega and F. J. García de Abajo, ACS. Phot. 4 (2017)

The combination of nonlinear and integrated photonics enables applications in telecommunication, metrology, spectroscopy, and quantum information science. Pioneer works in silicon-on-insulator (SOI) has shown huge potentials of integrated nonlinear photonics. However, silicon suffers two-photon absorption (TPA) in the telecom wavelengths around 1550 nm, which hampers its practical applications. To get a superior nonlinear performance, an ideal integrated waveguide platform should combine a high material nonlinearity, low material absorption (linear and nonlinear), a strong light confinement, and a mature fabrication technology. Aluminum gallium arsenide (AlGaAs) was identified as a promising candidate for nonlinear applications since 1994. It offers a large transparency window, a high refractive index (n≈3.3), a nonlinear index (n2) on the order of 10-17 m2W−1, and the ability to engineer the material bandgap to mitigate TPA. In spite of the high intrinsic nonlinearity, conventional deep-etched AlGaAs waveguides exhibit low effective nonlinearity due to the vertical low-index contrast. To take full advantage of the high intrinsic linear and nonlinear index of AlGaAs material, we reconstructed the conventional AlGaAs waveguide into a high index contrast layout that has been realized in the AlGaAs-on-insulator (AlGaAsOI) platform. We have demonstrated low loss waveguides with an ultra-high nonlinear coefficient and high Q microresonators in such a platform. Owing to the high confinement waveguide layout and state-of-the-art nanolithography techniques, the dispersion properties of the AlGaAsOI waveguide can be tailored efficiently and accurately by altering the waveguide shape or dimension, which enables various applications in signal processing and generation, which will be reviewed in this paper.

Photonic devices performing required temporal and spatial transformations of optical signals are of great interest for a wide range of applications including all-optical information processing and analog optical computing. Among the most important operations of analog optical processing are the operations of temporal and spatial differentiation. Various types of resonant photonic structures performing these operations were previously proposed, such as phase-shifted Bragg gratings and other multilayer structures, resonant diffraction gratings, and nanoresonators.
In the current work, we present an overview of our recent results dedicated to the design of resonant nanophotonic structures for optical implementation of various differential operators including integrated structures for Bloch surface waves and guided modes. A special attention is paid to a simple planar (integrated) optical differentiator consisting of two identical grooves on the surface of a dielectric slab waveguide (the details are presented in our recently published work [L. L. Doskolovich, E. A. Bezus, N. V. Golovastikov, D. A. Bykov, and Victor A. Soifer, “Planar two-groove optical differentiator in a slab waveguide,” Opt. Express 25(19), 22328–22340 (2017)]). The studied planar differentiator operates in reflection and enables temporal and spatial differentiation of optical pulses and beams propagating in the waveguide. The differentiation is associated with the excitation of an eigenmode localized at the ridge cavity located between the grooves. We show that by changing the groove length one can choose the required quality factor of the resonance (and, consequently, the linearity interval of the transfer function of the differentiator) in accordance with the width of the frequency or spatial (angular) spectrum of the incident pulse or beam. The presented numerical simulation results demonstrate high-quality spatial, temporal and the so-called spatiotemporal differentiation. The proposed differentiator may find application in the design of on-chip all-optical analog computing and signal processing systems.

In our work, we employ the resonant electromagnetic properties of III-V semiconductor nanowires to design building blocks for nonlinear all-dielectric metamaterials and devices. Contrary to widely used Si and Ge nanostructures, III-V materials, such as GaAs or AlGaAs, have a direct band gap and non-centrosymmetric crystal structure, which makes them promising for the development of nonlinear metamaterials. We developed an innovative approach to fabricate disk and rod nanoantennas by slicing bottom-up grown nanowires using a focused ion beam milling (FIB). The proposed method allows to significantly decrease the influence of the substrate on the electromagnetic field distribution inside the nanoantenna and it opens the possibility to use any substrate regardless of the nanostructure fabrication process. With this technique, we study the influence of geometry, design and crystal structure on the characteristics of all-dielectric nanoantennas. It offers unique opportunities to fabricate high-quality structures with variable radii, longitudinal heterostructures with lattice-mismatched materials, and structures with different refractive indexes and crystal phases that are not available in bulk materials.